The present invention is directed to an inductively coupled fluorescent discharge lamp, and more specifically to an amalgam support in such a lamp.
With reference to FIG. 1, a conventional inductively coupled fluorescent discharge lamp 10 may include a light transmissive envelope 12 having a re-entrant cavity 14 with an exhaust tube 16 and an excitation coil 18 inside the re-entrant cavity.
The light output of the inductively coupled fluorescent discharge lamp is dependent on the mercury vapor pressure within the lamp envelope. Alloys of low temperature melting metals that amalgamate with the mercury are often placed within such lamps to regulate the mercury vapor pressure within the lamp. When the lamp is turned on, the lumen output is significantly reduced until the amalgam is heated to a point where it releases sufficient mercury vapor to permit efficient lamp operation. This may require from several to many minutes depending on the lamp construction.
In order to decrease the time to reach efficient operation of the lamp, a starter amalgam may be placed inside the lamp envelope where it will be quickly heated. One technique is to place the amalgam on a closed conductive loop that completely circles the re-entrant cavity in order to allow the loop to couple to the inductive field to heat the amalgam on the loop, such as shown in FIG. 5 of U.S. Pat. No. 4,437,041. One of the problems with this technique is that coupling to the amalgam-bearing loop reduces the efficiency of the plasma coupling.
Another technique is to place the amalgam on a flag that is placed in or near the lamp discharge. The amalgam on the flag is heated by the discharge and releases mercury sufficient to improve starting characteristics of the lamp. As shown in FIG. 1 herein which is an embodiment of the lamp in U.S. Pat. No. 5,412,288, for example, the amalgam is on a wire screen flag 20 where the wire support structure for the flag may be snapped onto an extension of the exhaust tube 16. One of the problems with this technique is the cumbersome manufacturing steps needed to provide an attachment location on the surface of exhaust tube for the support for the amalgam-bearing flag.
FIGS. 2a-c depict three other examples of prior art amalgam support arrangements. FIG. 2a shows an embodiment of the lamp disclosed in U.S. Pat. No. 5,841,229 in which the support for the flag 20 is held by a coil 22 fit snuggly into the inner diameter of the exhaust tube 16. FIG. 2b shows an embodiment of the lamp similar to that disclosed in U.S. Pat. No. 4,622,495. Here, the support for the flag 20 is pressed into a bead of molten glass 24 attached to the outside of the re-entrant cavity. FIG. 2c shows a lamp with a glass sealing wire 26 held in place by pressing it into the glass during the mount forming operation. The flag 20 is welded on the glass sealing wire. Each of these arrangements is difficult to manufacture in an automated facility.
U.S. Pat. No. 4,105,910 discloses a wire ring for holding laminar segments of an amalgam in a wire mesh on a glass stem in a fluorescent lamp that is not inductively coupled. The process for manufacturing this multipart assembly is cumbersome and also difficult to adapt to automated manufacturing. See also, for example, U.S. Pat. Nos. 5,767,617 and 6,137,236.